1. Field of the Invention.
This invention relates to audio cables generally and, more particularly, to a novel audio signal cable in which the geometry of the conductors therein and the dielectric which separates them has been arranged to raise the capacitance and lower the inductance of the cable, therewith lowering its characteristic impedance to the same order as that of the load, typically 2 to 10 ohns.
2. Background Art.
Ever since the development of high fidelity stereo technology a great deal of effort has been directed towards eliminating sound distortion due to imperfections in microphones, amplifiers and loudspeakers. As the components have been improved, it has become increasingly important that the signal is transmitted unimpaired between amplifiers and speakers and this has required special attention to the construction and routing of speaker cables.
Most conventional cables, including loudspeaker cables, have a relatively high "characteristic impedance in the range of 50 to 100 ohms. The characteristic impedance of a signal transmission cable is independent of its length but depends on its construction and the mutual distance and kind of insulation used between the conductors.
In this context, it is a serious limitation of conventional cables that their characteristic impedance is much higher than the impedance of loudspeakers which is mostly in the range of 2 to 8 ohms. The ensuing problem is heard in reflections, due to impedance mismatch, which impair sound quality increasingly as cables get longer. Measurements indicate that this kind of signal distortion becomes notable at the high end of the audible field starting with speaker cables as short as 10 feet.
The resultant loss of fidelity is especially important in fast, transient signals which are impaired by a much slower rise time at the speaker than at the amplifier. In many cases, several speakers are connected in parallel to the same cable, further lowering the load and enhancing the impedance mismatch. In addition, in cases where the cable is left open, or almost open, e.g., connected to a high impedance headphone, the result is severe HF ringing.
The kind of distortion described in the above comes into play in complex stereo music signals by disturbing the phase relationship between signal components of different frequencies. The result is that the sound becomes diffuse and less distinct with increasing cable length. This effect should not be confused with the well known signal clipping.
Especially in stereo sound, fidelity is dependent on extremely small differences interpreted by the human ear to perceive the location of each instrument among a multitude of instruments, e.g., in a symphony orchestra. In this case, phase distortion will disturb the impression of being present in the concert hall.
In large audio speaker systems, e.g., cinema systems, often frequency adjustments are required of the individual channels in order to compensate for differences in cable length and thus to repair the before mentioned phase and frequency dependence. Such adjustment would not be required if speaker cables were designed to match the characteristic impedance of the speakers.
In addition, all audio amplifiers use negative feedback to control and stabilize the amplification ratio and power bandwidth. The load impedance has to be taken into account when the feedback loop is calculated and fine tuned for the desired frequency response. Using a speaker cable with the correct characteristic impedance will greatly reduce the variation in load impedance with frequency.
Another problem related to conventional twin lead cables is that they are relatively open to neighboring fields because of the distance between the conductors. The effect of this may be overplay between channels when cables are routed together, or line frequency hum picked up from adjacent power wiring. The kind of effects described may be avoided either by extensive cable shielding or separate routing, but either measure often adds considerably to installation costs.